This invention relates to an ester derivative useful as a liquid crystal material and a liquid crystal composition containing the ester derivative.
Liquid crystal displays are widely utilized for watches, electronic calculators, word processors, televisions and the like. Among various liquid crystal displays, a common one is the liquid crystal display in TN type (twisted nematic type) which utilizes optical anisotropy and dielectric anisotropy of a liquid crystal material.
The liquid crystal materials which are presently used for liquid crystal displays, such as a TN type, are required to have a wide liquid crystal temperature range, a rapid electro-optical response, a wide visual angle range, a low driving electric voltage, a chemical stability, an optical stability, and the like.
In order to obtain a wide visual angle range and a high contrast, it is necessary to optimize the retardation of the liquid crystal layer, i.e. .DELTA.n.d (.DELTA.n : birefringence of liquid crystal material, d: thickness of liquid crystal layer). In the liquid crystal displays to be put to practical use, the thickness of the liquid crystal layer is set in a restricted range, and accordingly, the optimization of .DELTA.n of the liquid crystal material is necessary.
The liquid crystal material is also desired to have a low viscosity for speeding up of the response. The relation between the rise time .tau. on at the time of impressing electric voltage and the decay time .tau. off at the time of breaking electric voltage is represented by the following formulas: EQU .tau. on=.eta..sub.ii d.sup.2 (.sup..epsilon. O.sup..DELTA..epsilon. v.sup.2 -K.pi..sup.2).sup.-1 EQU .tau. off=.eta..sub.ii d.sup.2 /.pi..sup.2 K
In the formulas, .eta..sub.ii is a parameter of viscosity, d is the thickness of the liquid crystal layer, .epsilon..sub.0 is the dielectric constant in vacuum, .DELTA..epsilon. is the anisotropy of the dielectric constant, V is the impressed electric voltage, K=K.sub.11 +(K.sub.33 -2K.sub.22)/4(K.sub.11 : spraying elastic modulus, K.sub.22 : twisting elastic modulus, K.sub.33 : bending elastic modulus). Accordingly, in order to raise the response speed, it is essential that the liquid crystal material has a low viscosity, i.e. a low viscosity liquid crystal compound is essential.
Moreover, the driving electric voltage depends upon the threshold voltage Vth, and a liquid crystal display can be driven by a lower electric voltage by reducing the threshold voltage Vth. Accordingly, a liquid crystal material capable of reducing the threshold electric voltage Vth is required.
There has not been found a liquid crystal compound which satisfies all of the above required properties in a single material. Accordingly, in order to comply with the requirement, several kinds of liquid crystal compounds having various properties were mixed with a non-liquid crystal compound and put to practical use.